1. Field of the Invention
The present invention relates to a method for producing a transparent polymer film. More precisely, the invention relates to a method for producing a transparent polymer film, which is free from problems of smoke emission and oily contamination and which enables easy control of retardation expressibility. The invention also relates to a transparent polymer film produced according to the production method, and to a retardation film, a polarizer and a liquid-crystal display device comprising the polymer film.
2. Description of the Related Art
Polymer films of typically cellulose esters, polyesters, polycarbonates, cycloolefin polymers, vinyl polymers and polyimides are used in silver halide photographic materials, retardation films, polarizers and liquid-crystal display devices. These polymers may be formed into films that are more excellent in surface smoothness and uniformity, and therefore their films are widely employed as optical films. For example, a cellulose ester film having a suitable moisture permeability may be directly stuck to a most popular polarizing film of polyvinyl alcohol (PVA)/iodine in an on-line process. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizer.
In case where a transparent polymer film is used for optical applications such as retardation films, retardation film supports, polarizer-protective films, and liquid-crystal display devices, its optical anisotropy control is an extremely important element in determining the performance (for example, visibility) of display devices. The recent demand for widening the viewing angle of liquid-crystal display devices requires retardation compensation enhancement, and it is desired to suitably control the in-plane retardation (Re, hereinafter this may be simply referred to as “Re”) and the thickness-direction retardation (Rth, hereinafter this may be simply referred to as “Rth”) of the retardation film to be disposed between the polarizing film and the liquid-crystal cell of liquid-crystal display devices. In particular, a transparent polymer film having a small |Rth|/Re, especially having |Rth|/Re<0.5 are not easy to produce; and it is desired to develop a simple method for producing it with ease.
For producing a polymer film, disclosed is a continuous production method comprising adhering a thermoshrinking film to a polymer film, heating and stretching it, and thereafter peeling the thermoshrinking film (for example, see JP-A 5-157911 and JP-A 2000-231016). According to the Examples in these references, it is clarified that the polycarbonate films produced according to the method satisfy the condition of |Rth|/Re<0.5. However, the method is problematic in that it consumes a large quantity of thermoshrinking films and that the quality of the obtained films is not uniform. The problem is especially remarkable in high-elasticity polymers such as cellulose esters.
On the other hand, for producing transparent polymer films for optical use, preferably employed is a solution casting method capable of providing films of good surface smoothness. In case where films are produced according to a solution casting method, a plasticizer is preferably added for facilitating high-speed film formation. When a plasticizer is added, then the solvent may be readily evaporated away within a short period of time during drying in film formation according to a solution casting method, and therefore the residual solvent amount in the polymer film produced may be reduced. However, an ordinary plasticizer in a transparent polymer film may cause unfavorable phenomena when the film is processed under a severe condition during its production, or may have some negative influences on the film. For example, when a transparent polymer film containing an ordinary plasticizer is processed at a high temperature, it may emit smoke or may be contaminated with oil. Accordingly, the transparent polymer film containing a plasticizer is naturally limited in point of its production condition and processing condition. On the other hand, it is known to use a high-molecular-weight plasticizer in a triacetyl cellulose ester film for photography; however, it is difficult to expect the applicability of the film for optical use through high-temperature treatment (see JP-A 5-197073).